Vacuum gauges are useful for a number of applications. In many industries certain processes require pressure measurements over a wide-range of pressures, such as, from about 1000 Torr to less than 10-9 Torr. Some semiconductor and electronic device manufacturing processes, for example, typically require accurate pressure measurements from ultrahigh vacuum to atmospheric pressures.
One type of vacuum gauge includes a metal filament (usually platinum) suspended in a tube which is connected to the system whose vacuum is to be measured. Connection is usually made either by a ground glass joint or a flanged metal connector, sealed with a gasket or an o-ring. The filament is connected to an electrical circuit from which, after calibration, a pressure reading may be taken. Such a gauge is commonly called a “Pirani” gauge, in reference to its initial developer. The operation of a Pirani gauge is based on heat transfer from the suspended heater, through a gas, and to a heat sink. The thermal conductance through the gas is a function of the gas pressure. Thus measurement of the thermal conductance through the gas allows the calculation of its pressure. In addition to the heat transfer through the gas, there are three other heat transfer mechanisms: (1) conduction through the mechanical supports, (2) convection by the gas, and (3) radiation.
FIG. 1, includes views (A)-(C), and illustrates an example of a prior art micro-Pirani sensor 100. The heart of this sensor 100 is the meandering electric filament heater 102 shown at the center of the structure in FIG. 1A. This filament 102 is imbedded in a thin insulating diaphragm 104 (e.g., SiN) with an open cavity below (not shown). This thin diaphragm 104 is both electrically and thermally insulating. The diaphragm 104 is connected to a substrate 106. View 1(B) shows the addition of a layer of bonding material 108, which can receive a cap 112 of heat-sink material.
With continued reference to FIG. 1, the electrical insulation can be used to eliminate leakage currents from the heater 102. The thermal insulation reduces the parasitic heat loss that occurs due to conduction in the plane of the diaphragm 104. Drawbacks of this SiN insulating diaphragm are (i) that it transfers considerable heat to the substrate at vacuum, (ii) it limits the thermal transfer through the gaseous molecular exchange, and (iii) it exhibits non-uniform temperature distribution at the filament area.